Objective assessment of respiratory function plays an important part in understanding the physiology and pathophysiology of the respiratory system. Lung function is an indispensable tool for diagnosis and monitoring of respiratory disease states in adults and older children, but has not gained wide acceptance in the management of infant lung disease. The primary difficulty in measuring infant lung function is the inherent lack of cooperation requiring assessment during sedated sleep and the use of a face mask. To date there is no widely applied technique for the measuring infant lung function in the unsedated infant.
Some methods of monitoring respiration in an infant have been developed for specific needs. Known devices for monitoring infant lung function can generally be classified as either invasive or non-invasive with respect to the infant's airway.
Invasive monitoring devices include pneumotacchographs which are connected to a sealed face mask, or spirometers similarly connected to a face mask. Traditional methods of volume and flow measurement with a face mask and/or pneumotachograph induce error through the effects of trigeminal stimulation, increased dead space and resistive loading. Face masks are poorly tolerated in unsedated infants and induce arousal especially in light sleep. They are completely impractical in the awake infant.
In addition it is technically difficult to maintain a seal with the infant for protracted periods of time, thereby limiting the ability to acquire data dynamically during unpredictable respiratory events such as apnoeas, sighs and hypopnoeas. These events are typically associated with desaturation and understanding respiratory dynamics that surround such events forms an important part of sleep medicine.
Non-invasive devices typically use bands to detect changes in chest and abdominal wall dimensions to monitor breathing. Such devices are easier to operate technically, and are less disturbing to the subject. However, these devices generally suffer from inaccuracy in measuring or interpreting lung volume changes.
Respiratory inductance plethysmography (RIP) has been used as one such non invasive measure of tidal volume and minute volume in infants but is compromised by complex and time consuming calibration techniques, though a simplified calibration has been recently disclosed. A pneumotachograph and face mask is still required to calibrate RIP to measure volume accurately and therefore can only be used in very young or sedated infants.
A fundamental problem with RIP is the approximation of the infant respiratory system as a two compartment model. In disease states chest wall motion is complex with subcostal and suprastemal recession being typical features. In infants with respiratory distress syndrome, chest wall recession in the inferior aspect of the chest may occur with expansion in the upper portion of the chest, and it is unlikely that a single RIP band can accurately measure thoracic volume changes in such situations. RIP has not been validated as a measure of tidal volume in infants with lung disease, and it is known to be inaccurate in infants under 1.5 kg presumably because of variable chest wall compliance.
Constant volume plethysmography involves the insertion of the infant in a sealed chamber and the application of a face mask to permit the infant to breathe fresh air and to remove expired air. Occlusion of the airway at the mask results in respiratory efforts by the infant for a small number of breaths. This, in turn, compresses and rarefies the gas within the chamber. By measuring the pressure changes, and knowing the volume of the chamber, Boyle's Law permits an estimate of the total gas within the infant's lung at the time of occlusion. However, the procedure is technically difficult, and the procedure is not suitable for protracted periods of time, e.g. during sleep.
Functional residual capacity (FRC) in infants is dynamically elevated above relaxed end expiratory volume using laryngeal and diaphragmatic breaking together with the commencement of inspiration before end expiration. The mechanism of FRC elevation therefore revolves around an interaction between the expiratory time constant and rate. FRC is usually reported as a single measurement and yet because of the way this volume is achieved by the infant it is likely that a range of volumes more accurately describes FRC. This is probably the case in REM sleep which is characterised by highly variable rate and varying degrees of laryngeal adductor activity.
FRC change with different sleep state has been investigated by different researchers with conflicting results. The concept of FRC variability was investigated using a respiratory jacket. This technique has not gained wide acceptance, presumably due to difficulties with calibration and fitting of the jacket. In order to interpret FRC measurements and indeed lung mechanics which are very dependent on lung volume, it is important to understand the natural degree of variability of FRC.
It is an object of this invention to provide an improved plethysmograph for measuring infant lung function non invasively during unsedated sleep without the need for a face mask.
It is yet another embodiment of this invention to provide an improved seal particularly, but not solely, suitable for use with the plethysmograph.